Important aspects of strategic leadership and management, including governance, remain inadequate.

The school’s senior leadership team, including the principal and the board of governors, have been ordered to “improve significantly the poor working relationships that exist between and among a significant minority of the teaching staff, the SLT and the governors”, and “improve the quality of the provision for CEIAG”.

However, inspectors pointed out that the standards attained by pupils at A-level have improved from below the Northern Ireland average for similar schools to above the NI average. ETI rated the overall quality of education as “satisfactory” — the fourth lowest of six possible grades, unchanged from an inspection in 2010.

Inspectors warned “in important aspects of management, including governance, there is insufficient improvement since the standard inspection and significant areas for improvement remain to be addressed”.

It is the second blow for the school this year after teacher Julie Muldrew successfully won a case against the board of governors that she had been “unlawfully and indirectly discriminated against” on gender grounds. She was awarded £5,000 compensation.

The Rev Dr Colin McClure, chairman of the board of governors, said in a statement to the Belfast Telegraph: "The principal, in accepting the report, commented on the difficult environment which exists in education, with schools facing financial constraints and reductions in staff.

"This inevitably causes conflict within any organisation and Larne Grammar School is no different. Staff are being asked to do more with less resources; it is natural that they feel aggrieved and under pressure.

"However, the board of governors and senior leadership team are committed to leaving no stone unturned in order to address the issues.

"External consultants Worklife Support will assist in improving working relationships within the school.

"However, as a school, we are disappointed that the considerable amount of work done since the inspection has not been acknowledged by the Inspectorate.

"Our key priority is, and will continue to be, the pupils who remain happy in school. The results they achieve, which are among the best in the province, particularly in terms of added value, should not be underestimated."

Larne Grammar is a co-educational voluntary grammar school that caters for 741 pupils aged 11-19. One of the proposals in the area plans published by the North Eastern Education and Library Board this month is for Larne High School and Larne Grammar School to establish a partnership, leading to the creation of one co-educational school for 1,200 pupils.